If the image formed by a lens for all positions of an object placed in front of it is always erect an diminished, what is the nature of this lens? Draw a ray diagram to justify your answer.

If the numerical value of the power of this lens is 10 D, what is its focal length in the Cartesian system?

If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, the nature of the lens is concave lens.

Ray diagram justifying the same is as follows:

Given: Power of the lens (P) is -10D (since concave lens hence negative sign is introduced).

Therefore focal length (f) is calculated in the following way:

Hence the focal length is -0.1m or -10cm.